Eukaryotic cilia and flagella are ancient cellular appendages that have been adapted for motile and sensory functions. Motile forms of these organelles are capable of propelling some cells like sperm and protozoa through a liquid environment while other cells like the ciliated trachea of man use the coordinated beating of many cilia to propel a liquid or mucous environment over the surface of the cells. Nonmotile cilia have been adapted to sense a wide range of stimuli. Classic examples include the photoreceptors which are highly modified cilia that can sense visible light and the olfactory cilia which are highly enriched in odorant receptors. Because of their important roles in both motility and sensory transduction, defects in cilia and flagella have been intimately linked with a number of human diseases including retinal degeneration, immotilie cilia and Kartagener's syndromes, male and female infertility, hydrocephalus and anosmia, Bardet- Beidl syndrome and one of the most commongenetic diseases in man, polycystic kidney disease. Focusing on how cells build these organelles, we study intraflagellar transport (IFT) which is required for the assembly and maintenance of these structures. IFT is characterized by the movement of protein particles along the long axis of the organelle, both out to the tip (anterograde IFT driven by kinesin-2) and back to the cell body (retrograde IFT driven by cytoplasmic dynein 1b/2). A primary function of IFT is to transport axonemal building blocks out to the distal tip which serves as the site of assembly for the organelle. The model organism for the study of IFT is the unicellular biflagellate green alga, Chlamydomonas reinhardtii. Biochemical analysis of the 17 proteins found in the Chlamydomonas IFT particles has begun to reveal their complex oligomeric organization. Continuing on our previous study of the IFT particles, our specific aims are: (1) to further characterize the architecture of IFT particles; to identify how the 17 proteins assemble into complexes; (2) to characterize the interaction of the IFT particles with the IFT motor proteins, kinesin-2 and cytoplasmic dynein 1b; (3) to directly visualize the transport of hypothesized IFT cargo including tubulin and radial spoke complexes; and (4) to address the function of separate IFT particle proteins. An important aspect of Aim 4 will involve the creation of 900 or more motility mutants; currently we lack mutants in more than 1/2 of the IFT particle proteins.